I thought the rules only apply if you post unsolicited referral links.
That's for PMs: if someone asks for it, sending referral links by PM are okay
No I think UserU is right.
According to mprep himself it's ok to post referral links in this situation.
That is to say, only the "spam" of referral codes is forbidden.
4. No referral code (ref link) spam. [1]
If you are posting a reply on a thread, and your answer is relevant. Would adding your referral link instead of the plain url be a problem?
For example, you are suggesting service a-dot-com as an answer's to OPs question. Would a ref link (a-dot-com?ref=my-ref) be allowed?
and is constructive (i.e. not just parotting content that was already posted),
it's allowed.